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The British Journal for the Philosophy of Science

Symmetries and Paraparticles as a Motivation for Structuralism

Adam Caulton, Jeremy Butterfield

Published Nov 24, 2011• 23 citations
Publisher

Abstract

This article develops an analogy proposed by Stachel between general relativity (GR) and quantum mechanics (QM) as regards permutation invariance. Our main idea is to overcome Pooley's criticism of the analogy by appeal to paraparticles. In GR, the equations are (the solution space is) invariant under diffeomorphisms permuting spacetime points. Similarly, in QM the equations are invariant under particle permutations. Stachel argued that this feature—a theory's ‘not caring which point, or particle, is which’—supported a structuralist ontology. Pooley criticizes this analogy: in QM the (anti-)symmetrization of fermions and bosons implies that each individual state (solution) is fixed by each permutation, while in GR a diffeomorphism yields in general a distinct, albeit isomorphic, solution. We define various versions of structuralism, and go on to formulate Stachel's and Pooley's positions, admittedly in our own terms. We then reply to Pooley. Though he is right about fermions and bosons, QM equally allows more general types of particle symmetry, in which states (vectors, rays, or density operators) are not fixed by all permutations (called ‘paraparticle states’). Thus Stachel's analogy is revived. 1 Introduction2 Structuralism Applied to General Relativity and Quantum Mechanics 2.1 Structuralism and individuality 2.2 The semantics of the structuralist 2.3 General relativity and Leibniz equivalence3 Anti-haecceitism and Quantum Mechanics 3.1 Pure and mixed states; permutations 3.2 Symmetrization and indistinguishability 3.3 Quantum statistics and the bad argument for anti-haecceitism4 The Generalized Hole Argument for Sets 4.1 Models and possible worlds 4.2 Permutations and permutes 4.3 State descriptions and structure descriptions 4.4 Theories; permutability, fixity, and general permutability 4.5 Stachel's argument for structuralism5 Fixed Theories and Metaphysical Under-determination 5.1 Pooley's objection; amending Stachel's premises 5.2 Is SP anti-haecceitistic?6 Superselection and Paraparticles 6.1 Quantum permutability 6.1.1 Symmetry types and symmetric operators 6.1.2 Statistics and symmetrization; superselection 6.1.3 The two arguments for quantum permutability 6.2 Quantum general permutability 6.3 Examples of quantum permutability 6.3.1 Two quantum coins, again 6.3.2 A generalized ray

DOI
10.1093/bjps/axr034

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This article develops an analogy proposed by Stachel between general relativity (GR) and quantum mechanics (QM) as regards permutation invariance. Our main idea is to overcome Pooley's criticism of the analogy by appeal to paraparticles. In GR, the equations are (the solution space is) invariant under diffeomorphisms permuting spacetime points. Similarly, in QM the equations are invariant under particle permutations. Stachel argued that this feature—a theory's ‘not caring which point, or particle, is which’—supported a structuralist ontology. Pooley criticizes this analogy: in QM the (anti-)symmetrization of fermions and bosons implies that each individual state (solution) is fixed by each permutation, while in GR a diffeomorphism yields in general a distinct, albeit isomorphic, solution. We define various versions of structuralism, and go on to formulate Stachel's and Pooley's positions, admittedly in our own terms. We then reply to Pooley. Though he is right about fermions and bosons, QM equally allows more general types of particle symmetry, in which states (vectors, rays, or density operators) are not fixed by all permutations (called ‘paraparticle states’). Thus Stachel's analogy is revived. 1 Introduction2 Structuralism Applied to General Relativity and Quantum Mechanics 2.1 Structuralism and individuality 2.2 The semantics of the structuralist 2.3 General relativity and Leibniz equivalence3 Anti-haecceitism and Quantum Mechanics 3.1 Pure and mixed states; permutations 3.2 Symmetrization and indistinguishability 3.3 Quantum statistics and the bad argument for anti-haecceitism4 The Generalized Hole Argument for Sets 4.1 Models and possible worlds 4.2 Permutations and permutes 4.3 State descriptions and structure descriptions 4.4 Theories; permutability, fixity, and general permutability 4.5 Stachel's argument for structuralism5 Fixed Theories and Metaphysical Under-determination 5.1 Pooley's objection; amending Stachel's premises 5.2 Is SP anti-haecceitistic?6 Superselection and Paraparticles 6.1 Quantum permutability 6.1.1 Symmetry types and symmetric operators 6.1.2 Statistics and symmetrization; superselection 6.1.3 The two arguments for quantum permutability 6.2 Quantum general permutability 6.3 Examples of quantum permutability 6.3.1 Two quantum coins, again 6.3.2 A generalized ray
This paper develops an analogy proposed by Stachel between general relativity (GR) and quantum mechanics (QM) as regards permutation invariance. Our main idea is to overcome Pooley's criticism of the analogy by appeal to paraparticles. In GR the equations are (the solution space is) invariant under diffeomorphisms permuting spacetime points. Similarly, in QM the equations are invariant under particle permutations. Stachel argued that this feature--a theory's `not caring which point, or particle, is which'--supported a structuralist ontology. Pooley criticizes this analogy: in QM the (anti-)symmetrization of fermions and bosons implies that each individual state (solution) is fixed by each permutation, while in GR a diffeomorphism yields in general a distinct, albeit isomorphic, solution. We define various versions of structuralism, and go on to formulate Stachel's and Pooley's positions, admittedly in our own terms. We then reply to Pooley. Though he is right about fermions and bosons, QM equally allows more general types of symmetry, in which states (vectors, rays or density operators) are not fixed by all permutations (called `paraparticle states'). Thus Stachel's analogy is revived.
Abstract Ontic structuralism or ontic structural realism in the philosophy of physics can be broadly considered as an interpretative strategy providing a set of conceptual and metaphysical tools—or, more ambitiously, an ontological framework—in order to account for central features of current fundamental physics. This article aims to review the main structuralist interpretative moves in the context of our two best fundamental physical theories of matter and spacetime, namely, quantum theory (quantum mechanics and quantum field theory) and general relativity. We highlight in particular the structuralist understanding of permutation invariance, entanglement and nonlocality in quantum theory, and of the dynamical features of spacetime, (gauge‐theoretic) diffeomorphism invariance and background independence in general relativity.
Quantum systems invariant under particle exchange are either Bosons or Fermions, even though quantum theory in principle admits more general behavior under permutations. But why do we not observe such paraparticles in nature? The analysis of this question was previously limited primarily to specific quantum field theory models. Here we give two distinct model-independent arguments that rule out parastatistics, i.e. fundamentally indistinguishable quantum systems transforming under higher-dimensional representations of the symmetric group, which draw on quantum information theory and recent research on internal quantum reference frames. First, we introduce a notion of complete invariance: quantum systems should not only preserve their local state under permutations, but also the quantum information they carry about other systems, in analogy to the notion of complete positivity in quantum information theory. Second, we demand that quantum systems are invariant under quantum permutations, i.e. permutations conditioned on values of permutation-invariant observables. For both, we show that the respective principle is fulfilled if and only if the particle is a Boson or Fermion. Our results show how quantum reference frames can shed light on a longstanding problem of quantum physics, they underline the crucial role played by the compositional structure of quantum information, and demonstrate the explanatory power but also subtle limitations of recently proposed quantum covariance principles.
We show that it is possible to formulate a consistent first-quantized theory of paraparticles, i.e., particles which are neither bosons nor fermions. We examine a number of properties of the theory and show that the formulation of Messiah and Greenberg in terms of generalized rays can be replaced by an equivalent formulation in which states are represented by rays in the usual way. We use this alternative formulation to establish some results of ordinary quantum mechanics. We examine in detail the consistency of the theory with the cluster law and show that paraparticles must have states associated with whole families of different permutation symmetries, according to the following rule: If a given particle has NNN-particle states associated with a given Young diagram, then it must have ($N\ensuremath{-}1$)-, ($N\ensuremath{-}2$)-,..., two-particle states associated with all Young diagrams which can be obtained from the first by successively removing squares. This gives rise to infinitely many different kinds of paraparticle, all with rather complicated properties.
The treatment of identical particles in quantum mechanics rests on two (related) principles: the spin‐statistics connection and the Symmetrization Postulate. In light of recent theories (such as q‐deformed commutators) that allow for ‘small’ violations of the spin‐statistics connection and the Symmetrization Postulate, we revisit the issue of how quantum mechanics deals with identical particles and how it supports or fails to support various philosophical stances concerning individuality. As a consequence of the expanded possibilities for quantum statistics, we argue that permutation symmetry is best formulated as a formal property of the state function describing the system of particles rather than as a property of the individual particles.1 Introduction2 Philosophical background2.1 Important terminology2.1.1 Identity2.1.2 Indistinguishability2.1.3 Indiscernibility2.2 When are particles indistinguishable?2.3 The Principle of the Identity of Indiscernibles and quantum mechanics2.4 The Principle of the Identity of Indiscernibles and logic2.5 Particle history2.6 Transcendental individuality3 Some quantum formalism3.1 The Principle of Permutation Invariance and the Symmetrization Postulate3.2 The configuration‐space approach3.3 Commutators and anticommutators, and identical particle statistics3.4 Q‐mutators4 Identical particle statistics: a holistic point of view5 Conclusions